In the offshore production of hydrocarbon fluids from a plurality of satellite wells in a subsea oil field, production fluid is often conducted along the seabed in flowlines to a central area or station where the production fluid may be conducted upwardly to a floating platform or vessel for initial processing. After processing, the processed fluid may be transferred to another area for storage or for further treatment. Very often, the processed fluid is returned to the seabed and conducted by another flowline to a storage facility, tanker, or onshore facility.
In the development of subsea oil fields of relatively shallow water depth, such as a few hundred feet, the transfer of production fluid from the seabed to a floating platform or vessel was not difficult. Subsea oil wells are being proposed for drilling and production at greater depths of water, such as 1,000 to 6,000 feet or more. The water environment for such depths poses problems not heretofore encountered.
Prior proposed riser constructions have included a single riser pipe extending from a seabed installation to a floating platform. Where a plurality of well holes were drilled in a subsea oil field, each well hole may have had its individual riser pipe extending to a floating platform. In other prior riser pipe constructions, an articulated multi-line riser was connected to a buoyant structure which was submerged at diver depth and which was connected to the top of a self-standing riser which extended downwardly to a foundation structure having a manifold serving as a collecting station for flowlines to the satellite wells.
A plurality of fluid conducting lines were included in the self-standing riser and the articulated portion of the riser above the buoyancy chamber included a plurality of flexible fluid conducting lines. The vertical forces resulting from the buoyant structure and acting on the manifold base was relieved by a weight carried by the riser just above the base. See British Patent No. 1,404,775.
An underwater buoy for a riser pipe to tension the pipe is shown in Canadian Patent No. 949,877. Other prior proposed solutions to the problems of applying a tensile force to the top of the riser pipe to prevent buckling have included the use of floatation jackets about the riser pipe at selected depths to relieve the tensioning system, and the provision of means for causing the buoy to rise at an angle and surface away from the vessel in the event the riser pipe breaks beneath the buoy. See U.S. Pat. No. 3,855,656.
In U.S. Pat. No. 3,729,756, a riser pipe is described in which a floatation collar is positioned around the riser pipe, the floatation collar comprising a plurality of plastic hollow spheres surrounded by a syntactic foam having a protective outer shell.